1. Field of the Invention
The present invention relates to an ultrasound transducer of a capacitance type and an ultrasound diagnostic apparatus.
2. Description of the Related Art
An ultrasound diagnostic method for applying an ultrasound into a body cavity and visualizing and diagnosing a state in a body from an echo signal of the ultrasound is prevalent. One of ultrasound diagnostic apparatuses used for such an ultrasound diagnostic method is an ultrasound endoscope. An ultrasound endoscope has an ultrasound transducer at a distal end of an insertion portion to be introduced into a body cavity. An ultrasound transducer has a function of converting an electric signal into an ultrasound and transmitting the ultrasound into a body cavity and receiving an ultrasound reflected in the body cavity and converting the ultrasound into an electric signal.
Conventionally, a piezoelectric element made of, e.g., a ceramic piezoelectric material such as PZT (lead zilconate titanate) has been mainly used as an ultrasound transducer. In recent years, an ultrasound transducer of a capacitance type (Capacitive Micromachined Ultrasound Transducer; hereinafter referred to as a c-MUT) manufactured using a micromachining technique has received attention. A c-MUT is disclosed in, e.g., Japanese Patent Application Laid Open Publication No. 2004-503313.
A c-MUT is configured to include a pair of flat electrodes (parallel plate electrodes) opposed to each other across an air and vacuum portion. The c-MUT transmits and receives an ultrasound in response to vibration of a film (membrane) including one of the electrodes.
Patent Document 1: Japanese Patent Application Laid Open Publication No. 2004-503313
At the time of transmission, the c-MUT vibrates the membrane in response to a change in electrostatic attraction between the pair of electrodes caused by application of a voltage signal at a predetermined frequency across the electrodes and transmits an ultrasound.
At the time of reception, since the c-MUT converts an ultrasound signal into an electric signal based on a change in capacitance between the pair of electrodes need to be charged in advance. It is thus necessary for the c-MUT to apply a DC bias voltage across the pair of electrodes at the time of reception.